In postage meters and other value printing devices a risk exists that an unauthorized user may insert an envelope or other workpieces into the housing within the meter to affect an impression of value known as "wiping-off". In all types of postage meters a security system to prevent unauthorized impressions from being made is important. This safeguard is needed to maintain the integrity of the meter and to provide a proper accounting of printed postage. This security system, however, in addition to being reliable, must be relatively usable and compatible with existing postage meter designs. The need for a reliable tamper-proof system exists in both postage meters having rotatable valve dispensing means and those using a die having substantially coplanar type.
It is known to provide various tamper-proof features in postage meters having a rotatable printing drum. In U.S. Pat. Nos. 2,828,591; 3,682,378; and 4,007,359 typical postage meters having mechanisms which rotate to print a postage impression of a selected value are described. In these prior art devices the drive gears and racks compel the presence of narrow spaces between the type wheels. It is in these spaces that the die protection means are provided. These protection means will project out from the face of the die at times during the printing cycle when it is desired that no impression be taken. Thus, envelopes or other workpieces cannot be fit into these spaces to obtain a bogus impression. In U.S. Pat. No. 2,829,591 the die protection means involves the use of thin leaf-like levers or blades which are sandwiched between the type wheels and have activating arms projecting to one side of the printing surface of the type wheels. These arms are retractable below this surface to permit printing at appropriate times. In U.S. Pat. No. 4,007,359 a rotatable drum postage meter is described which comprises a directly accessible hand manipulatively settable postage value selector means, a variable postage printing means and a postage registering means. The printing means and the registering means are only operable upon rotation of the drum from a rest position through a metering cycle. While these patents disclose effective die protection means, they all are limited to rotatable drum type postage meters.
Security features in a rotary meter are different from those in a platen meter. In a rotary meter the full die face is never exposed as in platen printing. There are no retractable mechanisms in a rotary meter (such as those disclosed in U.S. Pat. No. 4,007,359) which prevent access to the full die face.
In U.S. Pat. No. 2,795,186 a value printing device comprising a fixed substantially coplanar printing die is described. In this device there is provided a movable shroud which can be lowered to guard the printing die against taking unauthorized impressions at any time between legitimate printing operations. There are also described means for locking the shroud in guarding position except when the mechanism is operated in a normal manner to simultaneously print and register. This shroud in effect completely covers the face of the value printing die when the mechanism is not in an operating cycle. This security structure comprises a shroud overlying the guide table which serves in part as an inverted upper guide table. In home position the shroud depends a substantial distance below the surface of the printing die and forms with the table a guide chute into which a workpiece may be conducted without danger of smudging the same on the surface of the printing die and without conflicting with the edge of the platen. The shroud is supported for vertical reciprocating movement by means of four parallel links each pivoted at one end on a stud extending from the frame. Means is also provided for determining the lower limit of movement of the shroud and for preventing lifting of the shroud until the printing stroke has been commenced. This means includes a pair of slides, one on each side of the frame. The slide is mounted for sliding movement on two studs. The slide is slidable on these studs. Each of the slides has an irregularly shaped cam slot including a narrow throat portion which cooperates with a projection on the adjacent portion of the shroud. The throat portions of the slots are so configured as to prevent raising of the projections and hence of the shroud when the parts are in home position, whereby the shroud also performs the function of assisting in preventing the taking of unauthorized impressions from the value printing die. This prior art device involves completely covering the face of the printing die. This type of security system for value printing devices comprising fixed coplanar printing dies still allows one to insert an envelope or meter tape into the printing area and possibly wiping-off impressions. The only way to truly prevent wiping-off is to mechanically prevent access to the dies while the machine is at rest.